PROJECT SUMMARY Opioid misuse and dependence have become one of the largest public health crises in the US, with devastating socio-economic repercussions. In the attempt to curb this epidemic, NIH has recently issued the HEAL initiative to promote the development of novel and better strategies for OUD prevention and therapy. The goal of our team is to develop novel non-opioid drugs that may prevent or counter the abuse liability of opioids. To this end, the Peterson lab has recently developed a high-throughput zebrafish assay to screen for drugs that may reduce opioid self-administration. Drug screening with this paradigm revealed that one of the most effective molecules to suppress opioid seeking was finasteride (FIN), a drug approved for clinical use with good safety and tolerability. FIN inhibits 5?-reductase (5?R), the enzyme catalyzing the rate-limiting step in the synthesis of several neurosteroids (including allopregnanolone and other allosteric modulators of GABA-A receptors), as well as in the metabolism of testosterone and corticosterone. However, the specific changes in steroid profiles whereby FIN reduces opioid self-administration remain unclear. Over the past few years, the Bortolato group has studied the behavioral effects of FIN in rodent models of psychiatric disorders and found that this drug reduces the severity of impulsive behaviors and self-administration of other drugs of abuse. These studies have also shown that FIN?s effects are related to dopamine signaling, but do not affect the function of opioid receptors. Accordingly, our preliminary data show that FIN does not interfere with the analgesic properties of opioids in rat models. The exploratory collaborative studies proposed in this application are aimed at obtaining critical data on the translational power of our discoveries on the zebrafish model and ascertain the mechanisms whereby FIN reduces opioid self-administration. The two aims of this proposal will: 1) use a well-validated rat model to test whether opioid self-administration is reversed or prevented by FIN, and study potential sex differences in relation to these effects; and 2) use the high-throughput zebrafish model to identify which steroids (substrates and products of 5?R) are responsible for the effects of FIN on opioid self-administration. The results of the proposed studies will lead to future studies to validate the effects of FIN on multiple stages of opioid self- administration (also in rat models of chronic and neuropathic pain) and the steroid-based mechanisms whereby FIN modifies the rewarding effects of opioids. Given that FIN has been used in patients for more than 25 years and has a very good tolerability and safety profile, these studies may rapidly lead to clinical trials on its use as an adjunct treatment to opioid pain-killers that may reduce opioid abuse liability without interfering with the analgesic properties of these drugs.